Cholesteric liquid crystals have the property of maintaining several different optical states in the absence of an electrical field. Additionally, cholesteric liquid crystals can change optical states in response to applied electrical and/or thermal fields. Those properties make them useful in the development of field-stable, re-writable displays.
Cholesteric (chiral nematic) liquid crystals in a planar state are known to reflect circularly polarized light. The peak reflection wavelength is λ= nP0, and the band width is Δλ=ΔnP0, where P0 is the pitch,
            n      _        =                  1        2            ⁢              (                              n            e                    +                      n            o                          )              ,Δn=ne−no, ne and no are the extraordinary and ordinary refractive indices, respectively.
The pitch P0 can be adjusted by controlling the concentration c of the chiral dopant according to
            P      0        =          1              c        ·        HTP              ,where HTP is the helical twisting power of the chiral dopant. Thus, the peak wavelength λ= nP0 can be tuned to be in the infrared, visible, or ultraviolet spectrum. To reflect a short wavelength, such as a blue light, a short-pitch cholesteric liquid crystal is required, which in return requires a high concentration c, because for a given chiral dopant, the helical twisting power HTP is fixed. However, there is an upper limit to the concentration c of the chiral dopant. When its concentration gets too high, crystallization may occur, and other desired electro-optical properties of the host nematic liquid crystals may be lost. Another approach is to choose a high twisting power chiral dopant, but these are not readily available.
According to another approach as disclosed in U.S. Pat. No. 5,668,614, the wavelength of reflected light from cholesteric liquid crystals was tuned by photo-irradiation, which caused changes in the chirality of the tunable chiral material (TCM). This technique basically destroyed or altered the chirality of the chiral dopant. When the TCM and cholesteric liquid crystal of the same handedness were mixed together, the initial mixture was designed to reflect blue light. Upon irradiation with UV light or other high energy source, the TCM was destroyed, thus the chirality was reduced, and the wavelength of light moved from blue towards red. When the TCM and cholesteric liquid crystal of the opposite handedness were mixed together, the initial mixture was designed to reflect red light. Upon photo-irradiation, again, the TCM was destroyed, thus the effective chirality was increased due to the decrease in the opposite direction, and the wavelength of light moved from red towards to blue. However, the shortest wavelength that the mixture could achieve was limited by the chirality of the cholesteric liquid crystal without having the TCM. In either case, this technique did not increase the chirality of the chiral dopant. In addition the photo-irradiation usually caused adverse effects on other components of the cholesteric liquid crystals.